1. Technical Field
The present invention relates generally to compensating for memory effects of an amplifier.
2. Related Art
The function of an amplifier is to amplify a signal with as little signal distortion as is practical. An ideal amplifier is characterized as having a transfer function (input signal compared to output signal) which is completely linear with no transfer function discontinuities including memory effects, which is a type of hysterisis effect discussed in more detail below. Unfortunately, physical processes are seldom ideal and signal amplifiers are no exception. Amplifiers are specifically designed to operate as linearly and without memory effects as possible, but amplifier nonlinearities and memory effects are a reality in many amplifiers.
Because of natural and physical characteristics of amplifiers, amplified radio frequency (RF) output is often affected by hysterisis effects. Hysteresis is a distortion that is inherent to most amplifiers and affects the predistortion of amplifiers, which results in increased spectral regrowth and intermodulation. One type of hysterisis effect is known as a memory effect. Memory effects influence spectral regrowth and intermodulation distortion associated with amplifiers. The spectral regrowth and intermodulation distortion that is characteristic of memory effects of an amplifier are forms of signal distortion where extra frequencies are also transmitted. The transmission of extra frequencies can be power inefficient and cause interference to other RF systems. Memory effects are caused by signals affecting the physical properties of an amplifier such that the amplifier is residually affected by a previous signal when a present signal is being amplified. As an illustration of the above characteristics of amplifier memory effects, FIGS. 1 and 2 are provided.
FIG. 1 is an exemplary block diagram of a RF amplifier according to an exemplary embodiment of the invention. The typical RF amplifier system 100 of FIG. 1 includes a RF input 110 that is connected to an amplifier 120, the amplifier 120 outputs an amplified RF output 130. The amplifier 120 is a non-linear amplifier with memory effects.
FIG. 2 is an exemplary graph of a transfer function for an amplifier with memory effects according to an exemplary embodiment of the invention. The Y-axis represents the output voltage Vout of amplifier 120 where the amplifier 120 has memory effect characteristics. The output voltage Vout is plotted against the amplifier's input voltage Vin along the X-axis. As shown, the resultant amplifier transfer function 220 is shown as thick and curving downward as input voltage Vin increases and the resultant output voltage Vout fails to likewise increase at the same rate. The transfer function 220 is thick due to the memory effects of the amplifier 120. The amplifier transfer function 220 curves downward in large part due to gain compression and other hysterisis factors that introduce spectral regrowth effects and intermodulation distortion.